NITROGEN CREDITS IN SOYBEAN CORN CROP SEQUENCES ON 3 SOILS

Economic and environmental incentives to accurately predict corn (Zea mays L.) N requirements emphasize the need to assign appropriate N cre dits for soybean (Glycine max L.) in soybean-corn crop sequences. This study was conducted to determine corn response to N and N credits for soybean in crop sequences. The effects of corn-corn (CC), soybean-cor n (SbC), and soybean-corn-corn (SbCC) crop sequences and applied N (0 to 225 kg N ha-1) on corn grain yield, N uptake, and soil NO3 and NH4 concentrations were determined for 4 yr at three sites differing in cl imatic and soil characteristics. Four-year mean yields in SbC were 1.4 and 2.2 Mg ha-1 higher than in CC at two sites with nonirrigated silt loam soils, but yield effects due to crop sequence were small on an i rrigated sandy soil. Corn response to applied N varied markedly among the three sites and between years at the sites on silt loam soils. Mea n corn N uptake in SbC was higher (51 kg N ha-1) than in CC on silt lo am soils but not on the sandy soil. Soybean N credits estimated with a fertilizer replacement value (FRV) approach and from the difference i n N rates at maximum yield in SbC and CC sequences (DNM) differed mark edly among locations and years and ranged from -22 to 210 kg N ba-1. S oybean provided little N to subsequent crops on sandy soils due to pro bable loss of residue N through leaching prior to use by the following crop. On silt loam soils, crop sequence effects on yield and N uptake indicate soybean N contributions to subsequent crops; however, fixed value N credits or N credits based on N response data combined over ye ars will seldom accurately predict actual soybean N contributions. Sit e-specific diagnostic tests are needed to improve crediting of N suppl ied by soybean in crop sequences.